Ruthenium oxide-supported metal catalysts

The reduction of palladium oxide, rhodium oxide or ruthenium oxide gives the corresponding metal blacks generated by in situ hydrogenation in the reaction mixture. At present the use of these oxides, as well as Adam s catalyst, is not common because of the cost of the materials and the relatively large amounts which are required. These materials have been replaced by the more reactive and less expensive supported metal catalysts described in Chapter 13. 

Another ion exchange procedure involves the interaction of a metal acetylacetonate (acac) with an oxide support. Virtually all acetylacetonate complexes, except those of rhodium and ruthenium, react with the coordinatively unsaturated surface sites of 7 alumina to produce stable catalyst precursors. On thermal treatment and reduction these give alumina supported metal catalysts having relatively high dispersions. 38 Acetylacetonate complexes which are stable in the presence of acid or base such as Pd(acac)2, Pt(acac)2 and Co(acac)3, react only with the Lewis acid, Al" 3 sites, on the alumina. Complexes which decompose in base but not in acid react not only with the Al 3 sites but also with the surface hydroxy groups. Complexes that are sensitive to acid but not to base react only slightly, if at all, with the hydroxy groups on the surface. It appears that this is the reason the rhodium and ruthenium complexes fail to adsorb on an alumina surface. 38... 

The dual-state behaviour of RU-AI2O3 catalysts may also arise from metal-support interaction. In the oxidized state, the catalyst was more selective for nitrogen formation in NO reduction than when in the reduced state. It was also active for the water-gas shift reaction whereas the reduced form was rather inactive and differences were also observed for ammonia decomposition and the CO-H2 reaction. The more active form does not appear to contain ruthenium oxide the reduced catalyst may have been de-activated by reaction with the support and its transformation to the more active form by oxidation may involve surface reconstruction and/or destruction of the metal-support interaction. 

Indeed, there is a unity with the field of heterogeneous catalysis. As evidence of this, similar (or identical) Rh (C0)2 sites can be prepared either by CO chemisorption on preformed metal particles [69] or by decomposition of rhodium carbonyl clusters on the oxide surface [62-66]. Further evidence for this can be seen from the observation of metal carbonyl clusters under operating supported metal catalysts. For example, ruthenium catalysts for the conversion of synthesis gas to polymethylene [122] afford mixtures of cluster species at elevated temperatures (120°C) and pressures (1000 atm) [123]. One of these was Ru3(CO)i2 others appear to be ill-characterised. A similar observation has been recently reported for Ru/MgO and Os/MgO synthesis gas conversion catalysts [124]. On this basic support, two anionic clusters were isolated, viz. [Ru5C(C0)i5] and [OsiQC(CO)24] 7 which may be synthesised in solution by thermolysis in basic or reducing media. It is unclear whether these clusters are actually effecting the catalysis. They may instead, as highly stable species, be formed in a side reaction. 

Heterogeneous catalysts, either as metals or as metal oxides, are easier to separate from the effluent stream and when coated onto porous carriers are more active than homogeneous catalysts in promoting oxidation. Some examples of heterogeneous catalyzed systems operating at subcritical temperatures (WAO conditions) include the following ruthenium supported on cerium (IV) oxide, the most active metal catalyst among precious metals... 

The metal catalysts active for steam reforming of methane are the group VIII metals, usually nickel. Although other group VIII metals are active, they have drawbacks for example, iron rapidly oxidizes, cobalt cannot withstand the partial pressures of steam, and the precious metals (rhodium, ruthenium, platinum, and palladium) are too expensive for commercial operation. Rhodium and ruthenium are ten times more active than nickel, platinum, and palladium. However, the selectivity of platinum and palladium are better than rhodium [1]. The supports for most industrial catalysts are based on ceramic oxides or oxides stabilized by hydraulic cement. The commonly-used ceramic supports include a-alumina, magnesia, calcium-aluminate, or magnesium-alu-minate [4,8]. Supports used for low temperature reforming (< 770 K) are... 

Besides oxide supported Sn-Ru catalysts, carbon supported catalysts also find application in hydrogenation reactions. Sn Mossbauer spectroscopy was used to investigate the tin component of ruthenium and tin supported on activated carbon catalysts containing 2 wt. % ruthenium and having Sn/(Sn-f Ru) ratios between zero and 0.4. Four major components in the Sn Mossbauer spectra were attributed to both Sn(II) and Sn(IV) oxides and to Ru-SnOx species formed on the surface of ruthenium metal particles. In addition to this " Sn spectra reveal the presence of minor amounts of Ru3Sn7 alloy phase. ... 

Iron and its compounds (carbide, nitride), as well as ruthenium, cobalt, rhodium, and molybdenum compounds (sulfide, carbide), are used most frequently to produce high-molecular-weight hydrocarbons. Iron can be prepared as a high-surface-area catalyst (==300 m /g) even without using a microporous oxide support. 7-AI2O3, Ti02, and silica are frequently used as supports of the dispersed transition-metal particles. Recently zeolites, as well as thorium oxide and lanthanum oxide, have... 

Alumina-supported ruthenium catalysts were prepared by impregnating alumina (Aerosil, 200 m /g) with ruthenium chloride from its aqueous solution. The catalysts composition was Ru Al203 = 2 98 by weight. The catalyst precursors were dried overnight at 120°C in an air oven, and were then calcined at 450 °C for 2 h to form a supported metal oxide [12,13]. The catalysts were reduced in a hydrogen flow at 150°C and 300 °C for 1 h each, and at 400 °C for 2 h in series and then passivated. They were reduced again at 400 °C for 2 h in situ before the catalytic reaction. 

Many different catalysts have been used for catalytic hydrogenations they are mainly finely divided metals, metallic oxides or sulfides. The most commonly used in the laboratory are the platinum metals (platinum, palladium and, increasingly, rhodium and ruthenium) and nickel. The catalysts are not specific and may be used for a variety of different reductions. The most widely used are palladium and platinum catalysts. They are used either as the finely divided metal or, more commonly, supported on a suitable carrier such as activated carbon, alumina or barium sulfate. 

Gunnoe has also reported examples of catalytic aromatic alkylation using a ruthenium complex and olefins. With propylene and other terminal olefins, a 1.6 1 preference for anti-Markovnikov addition was seen. The proposed mechanism involved olefin insertion into the metal-aryl bond followed by a metathesis reaction with benzene to give the alkylated aromatic and a new metal-phenyl bond (Equation (26)). DFT calculations supported the proposed non-oxidative addition mechanism. The work was extended to include catalytic alkylation of the a-position of thiophene and furan. With pyrrole, insertion of the coordinated acetonitrile into the a-C-H bond was observed. Gunnoe has also summarized recent developments in aromatic C-H transformations in synthesis using metal catalysts. ... 

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